Electrical insulating oil, and to electrical apparatus incorporating them

ABSTRACT

An insulating oil which has good resistance to oxidation and which is also non-gassing and of low viscosity comprises from 150 percent (by weight) of a residually unsaturated alkene polymer, and correspondingly from 99-50 percent of a low viscosity hydrocarbon oil. The polymer is derived predominantly from an alkene or alkenes having from three to five carbon atoms and has an average molecular weight in the range 100-900, preferably in the range 250-500. A mixture of such alkene polymers may be used. The hydrocarbon oil is preferably a mineral oil, and has a viscosity of 30 centistokes or less at 20* C.

I United States Patent [151 3,668,128 Anderson 5] June 6, 1972 [54]ELECTRICAL INSULATING OIL, AND 3,573,209 3 1971 de Dryven .252/63 TOELECTRICAL APPARATUS 3,318,799 5/1967 Acker..... INCORPORATING THEM2,719,182 9/1955 Ross ..252/63 X 2,870,385 1/1959 Rice ..252/63 X [72]Inventor: John Kenneth Anderson, Bexley, England 2921 01 1 H 9 0ordeltmu 252 X Assigneez British Insulated Callendervs Robinson XLimited, London, England Primary Examiner-Robert F. Burnett [22] Filed:Jan. 2, 1970 Assistant Examiner-M. E. McCamish [21] APP] NW 461Attorney-Buell, Blenko and Ziesenheim [57] ABSTRACT [30] ForeignApplication Priority Data An insulating oil which has good resistance tooxidation and Jan. 9, 1969 Great Britain ..1,298/69 which is alsonomgassing and f km viscosity comprises from 1-50 percent (by weight) ofa residually unsaturated alkene [52] US. Cl ..252/63, 1 17/154, 117/201, polymer, and correspondingly from 99 50 Percent of a low 51174/25 203/14 viscosity hydrocarbon oil. The polymer is derived predomi-5 nantly from an alkene or alkenes having from three to five car- 1 gibon atoms and has an average molecular weight in the range 100-900,preferably in the range 250-500. A mixture of such alkene polymers maybe used. The hydrocarbon oil is [56] References cued preferably amineral oil, and has a viscosity of 30 centistokes UNlTED STATES PATENTSor less at 3,163,705 12/1964 Feick ..252/63 17 Claims, No DrawingsELECTRICAL INSULATING OIL, AND TO ELECTRICAL APPARATUS INCORPORATINGTHEM The present invention relates to electric insulating oils and moreespecially, but not exclusively, to oils based on selected and refinedmineral oils. The invention is especially concerned with oils for use inthe impregnation of the dielectric of power cables (chieflypaper-insulated power cables) and of other electrical apparatus, such ascapacitors and transformers.

In order to obtain a low power factor and a high degreeof resistance tooxidation, it is desirable to use a mineral oil having a low aromaticcontent. At the present time mineral oil having an aromatic content ofless than about 7 percent is not readily available, and it is preferablethat the aromatic content should be asrclose as possible to this figure,and preferably below it if and when such oil becomes available. However,it is found that mineral oils of low aromatic content have a tendency toevolve gas, predominately hydrogen, under the influence of an electricfield, contributing to the formation of gas-filled voids in whichdischarges occur, leading to degration and ultimately to completebreakdown of the insulation. Similar difiiculties arise with some typesof synthetic hydrocarbon oil.

Such gassing is reduced and preferably avoided by adding to the oil aconstituent which will react with and so absorb nascent hydrogen, andoils containing such a constituent are known, and will hereinafter bereferred to, as non-gassing. The added constituent will for conveniencebe referred to as the additive.

Hitherto, when an oil of low viscosity (e.g., below 40 centistokes at 20C.) was required, aromatic additives, conventionally napthalenicextracts of petroleum, have been used, with the result that the powerfactor of the oil was raised, and its oxidation resistance decreased,

In accordance with the present invention, an insulating oil which hasgood resistance to oxidation and which is also nongassing and of lowviscosity comprises from l-5O percent (by weight) of a residuallyunsaturated alkene polymer derived predominately from an alkene oralkenes having from three to five carbon atoms (or a mixture of suchalkene polymers) the or each polymer having an average molecular weightin the range l-900, and correspondingly from 9950 percent of a lowviscosity hydrocarbon oil which has a viscosity of 20 centistokes orless at 20 C., and would gas if used alone.

The hydrocarbon oil is preferably a refined mineral oil which ispreferably predominately napthenic in character or of mixednapthenic/parafi'mic composition, and preferably of low aromaticcontent. Naturally, a mineral oil having a low power factor and highoxidation resistance is preferred.

In addition to propene, butylenes and/or amylenes, the alkene polymermay include a small number of units derived from ethylene and/or fromhigher alkenes. Polymers derived predominately from butenes arepreferred.

The average molecular weight of the alkene polymer is preferably below700 and more especially in the range 250-500. An average molecularweight of about 350 is suitable.

The preferred amount of the additive depends, subject to satisfactorynon-gassing properties, upon it molecular weight and upon the viscositydesired. When the molecular weight of additive is in the preferred rangeof 250-500, addition of from -15 percent is suitable for oils for use inoil-filled cables having internal oil channels, and from 10-40 percentfor oils for use either in accessories of such cables or in pipe-typecables.

The invention includes electrical apparatus comprising a body of porousand/or laminated insulating material impregnated with the oilsdescribed; it further includes oil-filled paper insulated electriccables in which the paper is impregnated with the oils described.

The principal advantages of the insulating oils in accordance with theinvention may be summarized as follows:

i. the amount of additive required for any desired degree of gasabsorption is less than when conventional additives of the aromatic typeare used.

ii. the viscosity of the oil can be controlled at a low value;

iii. the additive has no appreciable deleterious effect and may on thecontrary have a beneficial effect on the oxidative stability orelectrical properties of the oil;

iv. theadditive is formed as aby-product in the manufacture of alkylenepolymers of high molecular weight, and is therefore relativelyinexpensive;

v. the low temperature hydraulic behavior of the oil is good, and inparticular its pour point is not unduly raised; and,

vi. under conditions of electric stress, the oil is less sensitive thatconventional, oils to the presence of traces of moisture.

In the following tables, oils A and D are examples of selectedrefinednaphthenic mineral. base oils, sold by the Burmah Oil TradingC0,, Dussek Division under the designations T3252 andT 3251-.respectively, oils B, B1 and E examples of oils in accordance with theinvention for oil-filled cables of the type having internaloil channelsand oils C and F examples of accessory oils in accordance with theinvention. The polybutene additive in oils B, C, E and F is a commercialpolybutene sold under the Trade Name PQLYVIS 00v and having an averagemolecular weight of 350 and a viscosity at 20 C. of 120 centistokes. Thepolybutene additive in Oil B1 is another commercial polybutene having anaverage molecular weight of 300.

TABLE I Oil A B Bl C Mineral Oil 100% 87% 70% Content Oil A Oil A Oil APolybutene Nil 10% 13% 30% additive content Viscosity (CS) at 20C. 5.56.0 6.0 l l 60C. 2.0 2.3 2.3 3.6 Specific Gravity at 0.86 0.86 0.86 0.85

20C. Flash Point (closedcup) I10 I10 H0 H0 C. (Typical Pour Point C. 8575 75 64 Gassing characevolving nonnon- Strongly ter in range evolvingevolving non- 0-l 30C. evolving TABLE II Oil D E F Mineral oil 90% 70%content D Oil Oil D Polybutene additive Nil 10% 30% content Viscosity(cS) at 20C. l4 17 27 60C. 4.5 5.2 6.2 Specific gravity at 20C. 0.880.87 0.86 Flash point (closed cup) "C. (Typical) 130 28 Pour Point C. 6562 -57 Gassing character evolving non- Strongly in range evolving non-0- l 30 C. evolving What I claim as my invention is: 1. An insulatingoil which has good resistance to oxidation and which is also non-gassingand of low viscosity comprising a. from l-50 percent by weight of atleast one residually unsaturated alkene polymer having an averagemolecular weight in the range lOO-900, and derived predominately from atleast one alkene having from three to five carbon atoms and b.correspondingly from 99-50 percent of a low viscosity hydrocarbon oilwhich has a viscosity of 30 centistokes or less at C. and would gas ifused alone.

2. An insulating oil as claimed in claim 1 in which the molecular weightof said alkene polymer is in the range 3. An insulating oil as claimed.in claim 2 in which the molecular weight of said alkene polymer is inthe range 250-500.

4. Electrical apparatus comprising a body of insulating materialimpregnated with the insulating oil claimed in claim 1.

5. An oil-filled paper insulated electric cable in which the paper isimpregnated with the insulating oil claimed in claim 1.

6. An insulating oil which has good resistance to oxidation and which isalso non-gassing and of low viscosity comprising a. from 1-50 percent byweight of at least one residually unsaturated alkene polymer'having anaverage molecular weight in the range 100-900, and derived predominatelyfrom at least one alkene having from three to five carbon atoms and b.correspondingly from 99-50 percent of a low viscosity refined mineraloil having a viscosity of 30 centistokes or less at 20 C.

7. An insulating oil as claimed in claim 6 in which said alkene isbutene.

8 An insulating oil as claimed in claim 6 in which the molecular weightof said alkene polymer is in the range 100-700.

9. An insulating oil as claimed in claim 8 in which the molecular weightof said alkene polymer is in the range 250-500.

10. An insulating oil as claimed in claim 9 suitable for use inaccessories of oil-filled cables having internal oil channels and foruse in pipe-type cables and comprising from 10-40 percent of said alkenepolymer.

11. An insulating oil as claimed in claim 9 suitable for use inoil-filled cables having internal oil channels comprising from 5-15percent by weight of said alkene polymer, and correspondingly from -85percent of said low viscosity refined mineral oil.

12. An insulating oil as claimed in claim 11 in which said alkenepolymer is a polybutene.

13. An oil-filled paper-insulated electric cable in which the paper isimpregnated with the insulating oil claimed in claim ll.

14. An insulating oil as claimed in claim 6 in which the refined mineraloil is predominately naphthenic in character.

15. An insulating oil as claimed in claim 6 in which the refined mineraloil is of mixed naphthenic/paraffinic composition.

16. Electrical apparatus comprising a body of insulating materialimpregnated with the insulating oil as claimed in claim 6.

17. An oil-filled paper insulated electric cable in which the paper isimpregnated with the insulating oil claimed in claim 6.

2. An insulating oil as claimed in claim 1 in which the molecular weightof said alkene polymer is in the range 100-700.
 3. An insulating oil asclaimed in claim 2 in which the molecular weight of said alkene polymeris in the range 250-500.
 4. Electrical apparatus comprising a body ofinsulating material impregnated with the insulating oil claimed inclaim
 1. 5. An oil-filled paper insulated electric cable in which thepaper is impregnated with the insulating oil claimed in claim
 1. 6. Aninsulating oil which has good resistance to oxidation and which is alsonon-gassing and of low viscosity comprising a. from 1-50 percent byweight of at least one residually unsaturated alkene polymer having anaverage molecular weight in the range 100-900, and derived predominatelyfrom at least one alkene having from three to five carbon atoms and b.correspondingly from 99-50 percent of a low viscosity refined mineraloil having a viscosity of 30 centistokes or less at 20* C.
 7. Aninsulating oil as claimed in claim 6 in which said alkene is butene. 8.An insulating oil as claimed in claim 6 in which the molecular weight ofsaid alkene polymer is in the range 100-700.
 9. An insulating oil asclaimed in claim 8 in which the molecular weight of said alkene polymeris in the range 250-500.
 10. An insulating oil as claimed in claim 9suitable for use in accessories of oil-filled cables having internal oilchannels and for use in pipe-type cables and comprising from 10-40percent of said alkene polymer.
 11. An insulating oil as claimed inclaim 9 suitable for use in oil-filled cables having internal oilchannels comprising from 5-15 percent by weight of said alkene polymer,and correspondingly from 95-85 percent of said low viscosity refinedmineral oil.
 12. An insulating oil as claimed in claim 11 in which saidalkene polymer is a polybutene.
 13. An oil-filled paper-insulatedelectric cable in which the paper is impregnated with the insulating oilclaimed in claim
 11. 14. An insulating oil as claimed in claim 6 inwhich the refined mineral oil is predominately naphthEnic in character.15. An insulating oil as claimed in claim 6 in which the refined mineraloil is of mixed naphthenic/paraffinic composition.
 16. Electricalapparatus comprising a body of insulating material impregnated with theinsulating oil as claimed in claim
 6. 17. An oil-filled paper insulatedelectric cable in which the paper is impregnated with the insulating oilclaimed in claim 6.